Hardware-in-the-loop (HIL) is a technology that is widely used for early testing and verification of the functionality of control units, e.g., in the automotive field. In this method, actual control units are tested in interaction with components simulated in real time to determine the extent to which they comply with the specified requirements. The environment of the control unit to be tested, consisting of actuators, sensors and the processes in the vehicle, is simulated in real time, and the behavior of the control unit is investigated in this simulated environment.
HIL simulation allows testing of control units in an early stage of development and testing of their response in the event of sensor defects, for example. If testing of control units in hybrid vehicles is to be conducted by HIL simulation, then it is useful to simulate the batteries as a system component. Batteries comprised of a plurality of single cells are used in hybrid vehicles. In particular, control units for battery control, so-called battery management systems (BMS), are tested by HIL simulation. The purpose of such a BMS is to monitor the cells in a cell array and thus ensure safe and reliable operation of the cell array for the longest possible time. For testing the BMS by HIL simulation, the behavior of a cell array should be simulated. For these reasons, models should be employed for simulation of single cells as well as for simulation of large cell assemblies.
In comparison with traditional battery models such as those used for on-board simulation, models for battery management systems should simulate the response of the battery as an interconnection of a plurality of single cells. In the single cell model, the cell voltage and state of charge and possibly also the temperature response of a battery cell are simulated. A typical cell response in different technologies such as Li ion, Ni-MH or lead may be taken into account. This includes differences in charging and discharging as well as the dynamic response in load tests and power loss due to gassing effects, for example. The model of the battery is then usually made up of a plurality of single cell models.
Single cell parameters and cell states such as the internal resistance of the initial state of charge should remain individually adjustable and the resulting cell voltages must also be made available to the BMS individually, for example, by a cell voltage emulator such as that described in the German patent application having the application number 102010043761.1. The currents then set by the BMS for charge balancing of the cells are also to be taken into account.
The electric terminal response of the battery is of particular interest in conjunction with HIL applications in the field of hybrid vehicles, electric vehicles or BMS.
For modeling the electric terminal response of cells, the approach of describing the terminal response through equivalent electric circuit diagrams is often chosen. This yields a good compromise between the complexity of the electrochemical processes taking place in a cell, on the one hand, and the essential simplifications, for example, to ensure real-time capability of the cell model, on the other hand. Another advantage of modeling of battery cells by means of equivalent electric circuits is the possibility of expanding the arrangement under consideration by adding additional components, such as power converters to the simulations.
Single-cell models for automotive simulation models such as those distributed by the applicant are composed of a cell voltage model and a model for the state of charge. The cell voltage model makes it possible to ascertain the parameters of individual physical effects such as the internal resistance, diffusion and the double layer capacitor. The state of charge model takes into account the charge current and the discharge current of the single cell as well as leakage currents such as those occurring due to gassing effects in charging NiMH cells. Against the background of such a single-cell model, a cell array of n cells may be assembled by connecting n single-cell models in a circuit. However, such a model is no longer easy to handle when there are a large number of single cells; beyond a certain number of single cells, it is also no longer real-time capable. This means that even when using specialized hardware for HIL simulations, the real-time requirements of the models can no longer be met. Testing of a control unit, which also tests the response time of a control unit, for example, therefore cannot be performed under conditions which reflect reality.
The possibility of simulating only one single cell and scaling the output variables by multiplying them times the number of cells n is not feasible for testing of BMS because in this case parameter controls and different states of charge of the single cells can no longer be modeled.